Electrical cable with foamed semiconductive insulation shield

ABSTRACT

An electrical power cable with a foamed, compressible, semiconductive insulation shield which serves as both a cushioning layer and an electrical shield.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electrical cables, with an improvedsemiconductive insulation shield and the method of making the same. Morespecifically, the invention is concerned with an electrical cable with afoamed semiconductive insulation shield which serves as both acushioning layer and an electrical shield. Preferably, the foamedsemiconductive insulation shield is a closed-cell foamed semiconductiveinsulation shield.

2. The Related Art

Electric power cables for medium and high voltage typically include acentral core electrical conductor of copper or aluminum, an overlayingsemiconductive conductor shield, an electrical insulation layer formedover the conductor shield, a semiconductive insulation shield and ametallic shield overlaying the insulation shield. Preferably, an overallplastic jacket is positioned radially external to said metallic shield.The thickness of each of these layers is determined by voltage ratingand conductor size and is specified by industry standards such as thosepublished by the Insulated Conductors Engineering Association (ICEA),the Association of Edison Illuminating Companies (AEIC), andUnderwriters Laboratories (UL). Electrical cable performance criteriaare specified and tested according to AEIC and ICEA standards. Theconductor shield is most often a semiconducting polymer extruded overthe electrical conductor. The insulation layer is usually athermoplastic or thermoset material such as crosslinked polyethylene(XLPE), ethylene-propylene rubber (EPR), or polyvinyl chloride (PVC).The insulation layer may include additives to enhance the life of theinsulation. For example, tree retardant additives are often added toXLPE to inhibit the growth of water trees in the insulation. Theinsulation shield is usually an extrudable semiconducting polymer. Theinsulation shield must have a smooth interface with the insulation layerand exhibit an acceptably low voltage drop through its thickness andeliminate discharge. The AEIC specifies that the insulation shield musthave a volume resistivity of less than 500 Ω·m (Ohms×meters) at 90° C.and 110° C. Insulation shields usually form a layer which is adhered tothe insulation layer, or for high voltage cables, bonded to theinsulation layer. The metallic shield overlaying the insulation shieldmay consist of, for example, a lead or aluminum sheath, a longitudinallyapplied corrugated copper tape with an overlapped seam or welded seam,helically applied wires (i.e. drain wires or concentric neutral wires),or flat copper straps. It is important that the insulation shield be inelectrical contact with the metallic shield. U.S. Pat. No. 5,281,757(hereinafter the '757 patent) and U.S. Pat. No. 5,246,783, the contentsof both of which are incorporated herein by reference, disclose examplesof electric power cables and methods of making the same.

There is sometimes a semiconducting tape layer interposed between theinsulation shield and the metallic shield. The purpose of this tape maybe for waterblocking, cushioning, or both. If for cushioning or bedding,this tape most often is employed in conjunction with a metallic shieldcomprised of lead or aluminum sheaths, copper tape with or withoutwelded seam or with sealed overlap longitudinally applied corrugatedcopper tapes as in the heretofore referenced '757 patent. The cushioningeffect of the tape layer eases the pressure on the metallic shield dueto the expansion and contraction of the electrical cable core resultingfrom varying load cycles on the cable. The use of cushioning or beddinglayers are known under concentric neutral wire metallic shields;however, due to the expansion and contraction of the electrical cablecore, the concentric neutral wires often indent the insulation shield.This indent is sometimes transferred to the insulation layer, causing adisruption of the cylindrical interface between the insulation shieldand the insulation layer. This disruption leads to higher electricalstresses as well as to detachments of the semiconductive insulationshield from the electric insulation layer, which may result in prematurefailure of the insulation layer and the cable. The cushioning layershereinbefore described add an expensive component to the cable and addan additional manufacturing step.

When splicing or terminating prior art electrical cables, the metallicshield is removed from the splice/termination area. Conventional spliceand termination sleeves have portions that compress around theinsulation shield. When the splice or termination is completed, a largevoid is present between the sleeve and the insulation shield because theinsulation shield does not compress. These voids, if not properly orcompletely filled with a grease, can cause failure of the splice ortermination due to partial discharge which will eventually erode theinsulation layer.

U.S. Pat. No. 4,145,567 to Bahder discloses an electric power cablewhich employs a semiconducting compressible layer of closed-cell foamedplastic extruded over the insulation shield and under a metallic shieldcomprised of a longitudinally folded tape with bonded or welded overlapseam. As the cable core becomes highly heated, it expands and increasesin cross-section. The compressible layer between the insulation shieldand the inside surface of the metal shield accommodates the expansion ofthe core by decreasing in radial thickness. When the cable core cools,the compressible layer expands again, so that it maintains contact withthe cable core and the metal shield at all times. In this way, thepressure exerted by the compressible layer against the insulation shieldand the metallic shield is sufficient to prevent any flow of fluidlengthwise of the cable if the metal shield becomes punctured bylightening or other cause. Examples given for Bahder's compressiblelayer are EPR which is either semiconducting when used with a coppermetallic shield or filled with high dielectric constant fillers such astitanium dioxide, barium titanate, or magnesium zirconate. Furthermore,according to Bahder extruding the compressible layer is an additionalmanufacturing step and the problem of voids in splices and terminationsis not alleviated. Moreover, the compressible layer disclosed by Bahderfunctions as a cushioning layer which is used in electric cableconstructions in addition to an insulation shield.

Document WO 99/33070 in the name of the Applicant describes the use of alayer of expanded polymeric material arranged in direct contact with thesemiconductive insulation shield of a cable, in a position directlybeneath the metallic screen of the cable, and possessing predefinedsemiconductive and waterblocking properties with the aim of guaranteeingthe necessary electrical continuity between the conductor and themetallic screen.

SUMMARY OF THE INVENTION

From the related art documents mentioned above it is apparent that therewas a technical prejudice in the field according to which a foamed layerwere to be considered unsuitable for being used as the semiconductiveinsulation shield of a cable since the presence of voids within thesemiconductive foamed layer was believed to be dangerous from theelectrical point of view. In fact all said documents disclose anelectrical cable comprising a compact, i.e. non-foamed, insulationshield which can be associated with a foamed layer for providing thecable with waterblocking and/or impact resistance properties.

Nevertheless, the Applicant perceived that a semiconductive foamed layercould be used as an insulation shield without running the risk thatelectrical failures (e.g. partial discharges) can occur. Furthermore,the Applicant perceived that said foamed, i.e. compressible, layer couldbe used as both an insulation shield and a cushioning layer suitable forelastically and uniformly absorbing the radial forces of expansion andcontraction of the cable due to thermal cycles thereof during use:

Therefore, the Applicant found an improved electrical power cable,preferably for medium or high voltage applications, comprising anelectrical central core conductor, an overlaying semiconductiveconductor shield, an insulation layer overlying the conductor shield, afoamed, compressible and semiconductive insulation shield, and ametallic shield overlying the foamed insulation shield. Preferably, thefoamed semiconductive insulation shield is a closed-cell foamedsemiconductive insulation shield. The foamed insulation shield serves asboth an insulation shield and a cushioning or compressible layer. Thus,for electrical cables with solid, bonded metallic shields, orlongitudinally folded and bonded overlap metallic tape, a cushioninglayer is provided by the insulation shield itself to compensate for theexpansion and contraction of the cable core with varying loads, therebyeliminating the need for separate tapes or cushion layers. Additionally,because the entire insulation shield is compressible, there will be lessvoid to fill compared to conventional insulation shields during splicesand terminations. That is, accessory reliability enhancement is achievedbecause the foamed insulation shield will conform to thesplice/termination sleeve such that only a small void remains betweenthe sleeve and the insulation shield. This small void is easier to fill.This significantly decreases the likelihood of failure due to partialdischarge and eventual erosion of the insulation layer. The foamedinsulation shield of the present invention can also be used underconcentric neutral wires to eliminate insulation layer indent. Becausethe foamed insulation shield of the present invention is the samethickness as conventional insulation shields, less material is used inits manufacture and therefore the cost of the insulation shield issignificantly less than conventional insulation shields. Additionally,because the foamed insulation shield of the present invention comprisesthe entire insulation shield, the additional manufacturing steps ofapplying tape layers, cushioning or compressible layers over theconventional insulation shield are eliminated.

Accordingly, the invention has numerous advantages and applications.Therefore, it is one object of the present invention to provide anelectrical power cable which incorporates a foamed insulation shieldwhich functions not only as an insulation shield but also as acushioning layer which allows for expansion and contraction of the cablecore during cable load cycles without putting undue stress on themetallic shield or on the underlying insulation layer.

It is a further object of the invention to eliminate the need for aseparate cushioning layer, such as semiconducting tape, between theinsulation shield and the metallic shield of an electrical power cable,thus decreasing the cost of the cable and eliminating a process step incable manufacture.

It is another object of the invention to provide an electrical powercable which eliminates insulation layer indent from concentric neutralwires.

It is a still further object of the invention to provide an electricalpower cable which reduces the voids present in splices and terminations.

It is a still further object of the invention to provide a method ofmaking an electrical power cable with a foamed insulation shield whereinthe conductor shield, insulation layer, and the foamed insulation shieldare triple-extruded into a pressure vessel (i.e. CV curing tube).

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the present invention will be apparentfrom the following detailed description of the presently preferredembodiments in conjunction with the accompanying drawings in which:

FIG. 1 is an illustration of an electric power cable of the presentinvention incorporating the foamed insulation shield under a metallicshield;

FIG. 2 is an optical micrograph of a cross-section of the foamedinsulation shield of the present invention made using an exothermicfoaming agent and a catalyst, when the nitrogen pressure used duringmanufacturing was 135 psi;

FIG. 3 is a graph showing the distance/temperature profile of thecontinuous vulcanization (CV) tube in Example 2;

FIG. 4 is an optical micrograph of a cross-section of the foamedinsulation shield of the present invention made using a hybridexothermic/endothermic foaming agent, cured in a steam CV tube;

FIG. 5 is a graph showing the distance/temperature profile of the CVtube in Example 3;

FIG. 6 is an illustration of a cable splice depicting a void spacebetween the termination sleeve and a conventional insulation shield; and

FIG. 7 is an illustration of a cable splice depicting the reduced voidspace between the termination sleeve and the insulation shield of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a shielded electrical cable 10 of the presentinvention comprises an electrical central core conductor 12, anoverlaying conductor shield 14, at least one insulation layer 16 formedover the conductor shield, a closed cell foamed semiconductiveinsulation shield 18 formed over and adhered to the insulation layer 16,and an outer metallic shield 22. Preferably, an overall plastic jacket(not shown) is included.

The central core conductor 12 may be a solid conductor as shown or itmay be stranded. The core conductor is preferably of aluminum or copper.The overlaying conductor shield 14 is preferably a semiconductingcrosslinked polymer. Suitable conductor shields are available from manycommercial sources as would be known to one skilled in the art. Theinsulation layer 16 is preferably an XLPE, a tree retardant XLPE(TRXLPE), an EPR, or an EPDM, all of which are crosslinked insulations,and are commercially available in the industry. The metallic shield 22may be a solid or bonded metallic layer of lead or aluminum or it may bea longitudinally folded copper or aluminum tape with an overlap seamwelded or sealed with an adhesive which allows the overlap seam to movewith variations in temperature as described in the '757 patent. Themetallic shield might also be helically applied concentric neutral wiresand/or copper tape. The overall plastic jacket (not shown) is preferablyan insulative thermoplastic polymer, for example, polyvinyl chloride(PVC) or a polyethylene (PE).

The foamed semiconductive insulation shield 18 is preferably comprisedof a base material, which may advantageously be one of a number ofcommercially available materials marketed for insulation shieldapplications, such as LE-315A supplied by Nova Borealis or HFDA-0693 orHFDC-0692 supplied by Union Carbide Corporation. It is preferable thatthe base material be comprised of a crosslinkable ethylene acetate suchas ethylene vinyl acetate (EVA), ethylene butyl acetate (EBA), orethylene ethyl acetate (EEA), with other additives such as processingaids, secondary resins, and chemical crosslinking agents and aids. Theinsulation shield base material must be filled with a conductive filler,such as carbon black, preferably in an amount from about 20% to about40% by weight, or in any event, in an amount sufficient for theinsulation shield to exhibit a volume resistivity of less than about 500Ω·m. The insulation shield is foamed, in general, by adding a chemicalfoaming agent to the base material prior to extrusion of the basematerial onto the insulation layer. The insulation shield is foamed suchthat the density reduction of the insulation shield is between about 10%to about 40%. Less than about 10% density reduction does not providemany benefits. Greater than about 40% density reduction, depending onthe materials employed, usually will result in a degradation of materialproperties needed for an insulation shield for electrical cables.

The chemical foaming agent is activated by heat from a continuousvulcanization (CV) process. The chemical foaming agent decomposes,releasing a gas, and thereby foams the insulation shield base material.The chemical foaming agent is selected such that its activation ordecomposition temperature is greater than the extrusion temperature sothat decomposition of the foaming agent occurs after extrusion andsubstantially simultaneous with the crosslinking of the conductorshield, the insulation layer and the insulation shield, as will bedescribed in more detail hereinafter. The chemical foaming agent may beof the endothermic or exothermic type or a hybrid endothermic/exothermicagent. The chemical foaming agents may be added directly to theinsulation shield base material and mixed before extrusion, or morepreferably the chemical foaming agents are supplied in a masterbach witha compatible carrier resin, preferably an EVA, an EBA, or an EEA andmixed with the insulation shield base material in an amount of fromabout 1% to about 8% by weight of the insulation shield base materialwhen mixed. It is important that the chemical foaming agent be welldispersed in the insulation shield base material. Good dispersion isdependent on the mixing and extrusion equipment and the relative meltflow indices (MFI) of the chemical foaming agent masterbach and theinsulation shield base material. In the examples which follow, gooddispersion of the chemical foaming agent in the insulation shield basematerial was achieved in a 120 mm diameter extruder with a length todiameter ratio of 20:1 utilizing a Barrier Flight Mixing Screw and achemical foaming agent masterbach resin with an MFI greater than that ofthe insulation shield base material. As is known in the art, gooddispersion is both a function of shear rates in the mixer and relativeviscosities of the components.

The choice of whether to use an endothermic, exothermic, or hybridchemical foaming agent may depend on the selection of the base materialfor the insulation shield and compatibility therewith, extrusionprofiles and processes, CV process and parameters, the desired amount offoaming, cell size, and structure, as well as other designconsiderations particular to the cable being produced. In general, givensimilar amounts of active ingredient, exothermic chemical foaming agentswill reduce density the most and produce a foam with larger cells.Endothermic foaming agents produce foams with a finer cell structure.This is a result, at least in part, of the endothermic foaming agentreleasing less gas and having a better nucleation controlled rate of gasreleases than an exothermic foaming agent. Thus, the resulting foam willhave an increase in the number of voids but a decrease in the size ofthe voids. This may, in some systems, provide a smoother interfacebetween the insulation shield and the insulation layer. Use of hybridfoaming agents results in a lower gas yield than an exothermic foamingagent and, therefore, smooth surfaces and a finer cell structure areproduced while at the same time retaining some of the benefits of anexothermic foaming agent. Representative foaming agents which may beused include CELOGEN® OT from Uniroyal Chemical, an exothermic foamingagent; azodicarboamide, an exothermic foaming agent; or one of thefollowing foaming agents marketed by Clariant of Winchester, Va.:HYDROCEROL® BIH 40 E, an endothermic foaming agent; HYDROCEROL® CT1267,a hybrid exothermic/endothermic foaming agent; HYDROCEROL® CT1271, ahybrid exothermic/endothermic foaming agent; HYDROCEROL® CT1555, ahybrid exothermic/endothermic foaming agent; HYDROCEROL® CT1557, ahybrid exothermic/endothermic foaming agent; HYDROCEROL® CT1376, anexothermic foaming agent; and HYDROCEROL® CT1542, an exothermic foamingagent. Considerations for selection of chemical foaming agents will bedescribed hereinafter. In some cases, a catalyst material may beincluded in the masterbach, especially when employing an exothermicfoaming agent. The catalyst material may be used to adjust thedecomposition temperature of the foaming agent. A suitable catalystmaterial is BIK® OT (registered trademark of Uniroyal Chemical).

The interface between the foamed semiconductive insulation shield andthe insulation layer is preferably substantially void-free. Thus, thefoamed semiconductive insulation shield preferably has a closed-cellstructure so as not to provide channels for water propagation betweenthe insulation layer and the insulation shield, for mechanical strength,to ensure electrical continuity, and to assist in forming a skin 24 atthe interface between the insulation shield 18 and the insulation layer16 as open cells or voids at the interface may result in partialdischarge and erosion of the insulation layer. The choice of chemicalfoaming agent, carrier, and processing conditions significantly affectthe formation of the skin or the smoothness of this interface also.

In accordance with the present invention, a preferred method of makingthe electrical cable with a foamed semiconductive insulation shield isdescribed. An electrical central core conductor is advanced on aconventional cable extrusion line through an extrusion crosshead. Asemiconductive conductor shield is extruded onto the electricalconductor. A crosslinkable insulation layer is extruded onto theconductor shield. A crosslinkable insulation shield material having achemical foaming agent incorporated therein is extruded over theinsulation layer. Preferably, the conductor shield, the insulation layerand the insulation shield material are triple-extruded. Alternatively,the conductor shield is extruded separately, and the insulation layerand insulation shield are co-extruded. The chemical foaming agent andthe extrusion temperature are selected so that the decompositiontemperature of the chemical foaming agent is greater than the extrusiontemperature. In this manner the decomposition of the foaming agent willnot occur during extrusion. After exiting the extruder, the electricalcable is advanced into a conventional CV tube. The CV tube is at anelevated temperature to supply heat in order to activate thecrosslinking agents in the conductor shield, the insulation layer and inthe insulation shield material. The heat of the CV tube must besufficient to both crosslink the conductor shield, the insulation layerand insulation shield and to decompose the chemical foaming agent.

The Applicant has found that the distance/time that the insulationshield material is maintained above the decomposition temperature in theCV tube is an important parameter for obtaining the desired densityreduction (and possibly surface quality) in the insulation shield due tofoaming. In one instance the Applicant has found that when theinsulation shield attains a temperature in the CV tube just above thefoaming agent's decomposition temperature, the density reduction of theinsulation shield due to foaming was increased from a 5% reduction to a26% reduction by increasing the time the shield was kept above thedecomposition temperature by about 1.5 times, from 5.8 minutes to 8.5minutes. Keeping all other parameters constant, the Applicant has foundthat reducing the decomposition temperature of the foaming agent,decreasing the line speed to increase this time, or alternating thetemperature profile of the CV tube to increase this time, will each aidin obtaining the desired density reduction. Further, while, for example,the decomposition temperature may be around 200° C. [˜390° F.] (orsignificantly less such as about 160° C. [˜320° F.] depending on thefoaming agent used) many of the foaming agents suitable for the presentinvention have a recommended processing temperature range of 210° C. to240° C. [˜410° F. to 464° F.] (or more, again depending on the foamingagent used) to achieve optimum gas yield. Thus, it was found that evenif linespeed were increased, thereby decreasing the time the insulationshield was above the decomposition temperature to about 3 minutes,satisfactory results could still be obtained by altering the tubetemperature profile such that the insulation shield was in therecommended processing temperature range of the foaming agent for morethan one minute.

In a dry cure CV line, the CV tube is usually at a temperature of about400° C. [750° F.] and pressurized with nitrogen gas to about 135 psi toabout 150 psi. In a steam-cure CV line, the CV tube is pressurized withsaturated steam to a range of about 200 psi to about 260 psi at atemperature of about 193° C. [380° F.] to about 210° C. [410° F.]. Inthe case of using superheated steam, the CV tube is electrically heatedto about 380° C. [715° F.] with the tube pressurized with steam to about150 psi to about 200 psi. The CV tubes are pressurized in order toprevent foaming of the insulation layer due to the decomposition of thecrosslinking agent when crosslinking. Thus, foaming the insulationshield in the CV line is counterintuitive. The chemical foaming agentdecomposition, crosslinking, and foaming of the insulation shield occurin the CV tube substantially concurrently. The decomposition of thefoaming agent is preferably complete before the crosslinking of theinsulation shield is complete. The cable is then cooled and a metallicshield may be applied either on the same manufacturing line or, mostoften, in a separate operation.

Generally in the art, foaming of polymers is done at ambient pressure orin a vacuum. Thus, decomposition of the chemical foaming agent under thehigh pressure of the CV tubes as in the present invention is unique inthe art. While the foamed insulation shield could be applied over theinsulation layer in a separate extrusion operation, thereby eliminatingthe complications of decomposing the chemical foaming agent under thepressure of the CV tube, this separate additional extrusion step wouldincrease the cost of manufacturing, could promote defects at theinsulation layer/insulation shield interface, and defeat one of theobjects of the present invention; that is, to provide an electricalpower cable which incorporates an insulation shield that also functionsas a cushioning layer, thus decreasing the cost of the cable andeliminating a process step in cable manufacture.

Successful practice of the present invention was accomplished usingexothermic, endothermic, and hybrid chemical foaming agents in anEVA-based insulation shield base material. The following examplesprovide further details on the preparation of cables of the presentinvention.

The preferred insulation shield material (as used In Examples 1–4) is anEVA-based material having a primary resin of EVA with a 45% vinylacetate (VA) content in an amount of about 45% to about 55% by weight ofthe insulation shield material. A secondary resin of nitrile rubber(NBR) with an acrylonitrile (ACN) content of about 33% in the amount ofabout 10–20% by weight is included. Carbon black in an amount sufficientto make the insulation shield material semiconductive, typically about20–40%, is included. Other additives, typical to insulation shieldmaterials such as antioxidants, processing aids and crosslinking agentsin amounts less than about 5% each are also included. However, it is thepolymer system, the EVA with about 45% VA content, and NBR with 33% ACNcontent that makes this insulation shield material preferred with thespecific chemical foaming agents employed. When other chemical foamingagents contained in various other masterbaches are employed, a differentpolymer system for the insulation shield may be the preferred. Theaforementioned base material for the insulation shield of Examples 1–4was used initially because it was thought that modifications to the basematerial may be necessary to produce good foamed insulation shields.Surprisingly, this was not the case. It was found that adhering to theselection criteria of material for the chemical foaming agent and itsmasterbach, in relation to the base material employed, was the greatestdeterminant of success, along with the processing conditions.

EXAMPLES 1–2

Semiconductive insulation shields according to the present invention,employing exothermic chemical foaming agents, were extruded along withthe conductor shield and insulation layer onto No. 1/0 American WireGauge (AWG) conductor and foamed and crosslinked in a nitrogen gas (N₂)environment at elevated temperature and pressure. Table 1 gives theformulations and process conditions for Examples 1–2.

TABLE 1 CV Curing Foaming Insulation Shield Process Linespeed ExampleAgent/Type Composition Insulation Conditions (fpm) 1 Celogen OT 100 pphrEVA Base Resin TRXLPE N₂ at 135 psi and 35 Exothermic and additives(carbon black, tube zone Decomp. Temp: chemical crosslinking agent,Temperatures: 175° C.–220° C. processing aids); 2.52 pphr750/750/750/725/ (347° F.–428° F.) Celogen OT 1.05 pphr BIK 725/700° F.OT catalyst 2 Hydrocerol 96% by weight EVA Base EPR N₂ at 135 psi and 54CT1376 Resin and additives (carbon tube zone Decomp. Temp: black,chemical crosslinking Temperatures: 190° C. (374° F.) agent, processingaids): 4% 725/725/700/700/ by weight CT1376 (40% 675/650° F. Activecontent in EVA carrier Notes: pphr = parts per hundred rubber fpm = feetper minute

Examples 1 and 2 exhibited a density reduction of 32% and 20%respectively. Example 1 was further processed by helically applying sixNo. 14 AWG copper concentric neutral wires and extruding over thesewires an overall plastic jacket. It was observed that the concentricneutral wires created an indent in the foamed insulation shield;however, importantly, the indent did not transfer through to theinsulation layer. Further, upon application of heat, the indentdisappeared.

FIG. 2 is a cross-sectional view of the foamed insulation shield 18 ofExample 1, which shows more closely the closed-cell structure of thefoamed insulation shield 18 and the “skin” 24 which formed at theinsulation shield/insulation layer interface.

As seen in Table 1, the foamed insulation shield of Example 1 included acatalyst in order to lower the start of decomposition of the chemicalfoaming agent by approximately 15° C. (27° F.), from about 190° C. (374°F.) to about 175° C. (347° F.). Other experimental cables made with thesame process conditions and foamed insulation shield composition asExample 1, except without the catalyst, showed inferior surface qualityand inferior electrical performance as compared to the cable ofExample 1. The results of lowering the decomposition temperature of thefoaming agent in Example 1 demonstrates the importance and relationshipbetween the decomposition temperature of the foaming agent and theprocess conditions in the CV curing tube in obtaining a satisfactoryfoamed insulation shield.

The cable of Example 1 was subjected to the standard qualificationtesting for electric power cable performance as specified by AEIC. Thesetests included: volume resistivity, high voltage time testing analysis(HVTT), and partial discharge (PD) analysis. Example 1 passed the AEICspecification for volume resistivity, with a value of 2 Ω·m at 90° C.and 0.45 Ω·m at 110° C., well within the AEIC requirement of <500 Ω·m at90° C. and 110° C. The cable of Example 1 also passed the AEICspecification for HVRT, with a value of 700 volts/mil (V/mil)unconditioned and 1200 V/mil after conditioning at 100 hours at 90° C.The AEIC specification for partial discharge is: PD<5 picoColumbs (pC)at 4 Vg (4 times the rated voltage to ground). Example 1 had aninception level of PD at 20 kV and at 4 Vg (in this case 35 kV) of 5 pC.

The cable of Example 2 was observed to have superior surface quality.The foamed insulation shield of the cable of Example 2 was subjected totesting for insulation shield physical properties at original and aged 7days at 121° C. and 136° C.; hot creep/set at 150° C.; bond strength atambient temperature, −10° C., and +40° C.; and field strippability at+40° C. The results of these tests demonstrated that the insulationshield of Example 2 met and/or exceeded applicable AEIC/ICEA industryspecifications.

FIG. 3 shows the distance/temperature profile of the CV tube for Example2. It can be seen that the insulation shield obtained a temperatureabove the decomposition temperature of the foaming agent for about 55meters, or 3.33 minutes, based on the linespeed of 54 fpm in Table 1.The time the insulation shield was in the recommended working range ofthe foaming agent for optimum gas yield was about 1.2 minutes.Lengthening this time by reducing the decomposition temperature of thefoaming agent with the addition of a catalyst, slowing down thelinespeed, or by increasing the temperatures of the latter heating zonesof the tube would all help increase the density reduction if desired.

EXAMPLE 3

A semiconductive insulation shield according to the present invention,employing a hybrid exothermic/endothermic chemical foaming agent, wasextruded along with a conductor shield and an insulation layer onto No.1/0 AWG conductor (approximate equivalent of 53.49 mm² metricconductor), and foamed and crosslinked in a steam environment atelevated temperature and pressure. Table 2 gives the formulations andprocess conditions for Example 3.

TABLE 2 CV Curing Foaming Insulation Shield Process Linespeed ExampleAgent/Type Composition Insulation Conditions (fpm) 3 Hydrocerol CT127196% by weight EVA EPR Steam at 203 psi 6.6 Hybrid: Base Resin and andtube Endothermic/ additives (carbon temperature Exothermic black,chemical zones 1–4 at Decomp. Temp: crosslinking agent, 715° F. 190° C.processing aids); 4% by weight CT 1271 (70% Active content in EVAcarrier)

The foamed insulation shield of Example 3 achieved a density reductionof about 28% and had very good surface quality. When tested for partialdischarge according to AEIC requirements, Example 3 exhibited less than2 pC at 52 kV, which meets AEIC standards. Example 3 demonstrates thatthe foamed insulation shield of the present invention may be achieved onsteam cure CV lines.

FIG. 4 shows in cross-section the cell structure and skin of the foamedinsulation shield of the present example. FIG. 5 shows thedistance/temperature profile of the CV tube for Example 3. It is notedthat the insulation shield was just above (200° C.) the decompositiontemperature of the foaming agent (190° C.) for 11.6 minutes, based onthe linespeed in Table 2, resulting in a 28% density reduction of theinsulation shield.

EXAMPLE 4

A semiconductive insulation shield according to the present invention,employing a hybrid exothermic/endothermic chemical foaming agent, wasextruded along with a conductor shield and insulation layer onto No. 1/0AWG conductor, and foamed and crosslinked in a N₂ environment atelevated temperature and pressure. Table 3 gives the formulations andprocess conditions for Example 4.

TABLE 3 Foaming Agent/ Insulation Shield CV Curing Process LinespeedExample Type Composition Insulation Conditions (fpm) 4 Hydrocerol 96% byweight EVA EPR N₂ at 135 psi and 35 CT1271 Base Resin and tube zoneHybrid: additives (carbon temperature Endothermic/ black, chemical725/725/700/700/675/ Exothermic crosslinking agent, 650° F. Decomp.Temp: processing aids); 4% 190° C. by weight CT1271 (70% Active contentin EVA carrier)

The foamed insulation shield of Example 4 achieved a density reductionof about 32%. The cable of Example 4 was tested according to AEICrequirements and successfully complied with those requirements. Thetested cable passed wafer boil testing as specified in AEIC StandardCS6–94 Section G.2 which indicated the foamed insulation shield had beeneffectively crosslinked. The electrical testing data of the cable allmet the AEIC specifications including those from the alternating current(AC) withstand, partial discharge, dissipation factor, hot impulse, andvolume resistivity at room temperature, 90° C. and 130° C. The ambientbond strength, elongation at break in the original and aged states, andshield removability ranging from room temperature to +40° C., were alsofound to be well above the respective industrial requirements.

The cable of Example 4 experienced the same CV tube distance/temperatureprofile as shown in FIG. 3 for Example 2. However, because of the slowerlinespeed of the present example, the insulation shield was above thedecomposition temperature of the foaming agent for 5.1 minutes and inthe high end of the working temperature range of the foaming agent foralmost 2 minutes.

EXAMPLE 5

A semiconductive insulation shield according to the present invention,employing an endothermic chemical foaming agent, was extruded along witha conductor shield and insulation layer onto No. 1/0 AWG conductor,foamed and crosslinked in a nitrogen gas environment at elevatedtemperature and pressure. Table 4 gives the formulations and processconditions for Example 5. HFDC-0692, available from Union Carbide, wasused as the base material for the insulation shield.

TABLE 4 CV Curing Foaming Agent/ Insulation Shield Process LinespeedExample Type Composition Insulation Conditions (fpm) 5 Hydrocerol CT126796% by weight HFDC- TRXLPE N₂ at 135 psi and 40 Endothermic 0692(EVA-based tube zone insulation shield temperatures: material)750/750/750/725/ 4% by weight CT1267 725/700 F. (60% Active content inEVA carrier)The foamed insulation shield of Example 5 achieved a density reductionof about 10% and had good surface quality. This example shows how theuse of a purely endothermic foaming agent does not result in as great adensity reduction of the insulation shield with similar active contentof chemical foaming agent.

Furthermore, the cable of the present invention with a foamed insulationshield enhances accessory reliability as illustrated in FIGS. 6 and 7.Referring to FIG. 6, when an electrical cable 10 having a conventional,non-foamed insulation shield 30 is terminated or spliced, a large voidarea 45 between the insulation layer 16 and the splice or terminationsleeve 40 results as the insulation shield 30 has compressionresistance. These large voids are normally filled with grease; however,it is difficult to fill the void completely. Because of this, the voidscan potentially cause failure of the termination or splice due topartial discharge which will eventually erode the insulation. Using thefoamed insulation shield 18 of the present invention as illustrated inFIG. 7, however, the insulation shield 18 compresses under the sleeve 40when terminating or splicing the cable 10 and will substantially conformto the sleeve 40. Therefore, a significantly smaller void area 48between the insulation layer 16 and the sleeve 40 results. This smallvoid is more easily filled and requires less grease. The ability of thefoamed insulation shield 18 to deform under pressure, such as thatpressure which results from sleeve 40, allows enhanced reliability ofterminations and splices to cables of the present invention.

1. An electric power cable, comprising: a conductor; a semiconductiveconductor shield overlaying said conductor; a crosslinked insulationlayer formed over said conductor shield; and a foamed crosslinkedsemiconductive insulation shield positioned over and adhered to saidinsulation layer, wherein foaming of said foamed insulation shield isobtained after extrusion of said insulation shield onto said insulationlayer.
 2. The electric power cable of claim 1, wherein an interfacebetween said insulation layer and said foamed insulation shield issubstantially void free.
 3. The electric power cable of claim 1, whereinsaid foamed insulation shield has a closed cell structure.
 4. Theelectric power cable of claim 3, wherein said insulation shield iscomprised of a base material comprised of a crosslinkable ethyleneacetate selected from the group consisting of EVA, ERA, and EBA.
 5. Theelectric power cable of claim 4, wherein said chemical foaming agent iscomprised of a masterbach.
 6. The electric power cable of claim 5,wherein said masterbach is comprised of a carrier selected from thegroup consisting of EVA, ERA, and EBA, and an active chemical foamingingredient.
 7. The electric power cable of claim 6, wherein said carrierhas a MFI higher than that of said insulation shield.
 8. The electricpower cable of claim 5, wherein said chemical foaming agent comprisesfrom about 1% to about 8% by weight of said insulation shield.
 9. Theelectric power cable of claim 1, further comprising a metallic shieldoverlaying said foamed insulation shield.
 10. The electric power cableof claim 1, wherein foaming of said foamed insulation shield is obtainedby: adding a chemical foaming agent having a decomposition temperatureto said insulation shield prior to extrusion; and decomposing saidchemical foaming agent at greater than atmospheric pressure afterextrusion of said insulation shield onto said insulation layer.
 11. Theelectric power cable of claim 10, wherein said insulation shield iscomprised of a base material comprised of a crosslinkable ethyleneacetate selected from the group consisting of EVA, EBA, and EEA.
 12. Theelectric power cable of claim 1, wherein said chemical foaming agent isselected from the group consisting of exothermic foaming agents,endothermic foaming agents, and hybrid exothermic/endothermic foamingagents.
 13. The electric power cable of claim 1, wherein said chemicalfoaming agent is an exothermic foaming agent and wherein said pressureis greater than or equal to about 135 psi.
 14. The electric power cableof claim 13, wherein a catalyst is added to said insulation shield priorto extrusion onto said insulation layer.
 15. The electric power cable ofclaim 1, wherein foaming of said insulation shield causes from about 10%to about 40% density reduction of said insulation shield.
 16. A methodof producing an electrical power cable, comprising: advancing anelectrical conductor through an extrusion crosshead; extruding asemiconductive conductor shield over the electrical conductor; extrudinga cross-linkable electrical insulation layer over the conductor shield;extruding a semiconductive, crosslinkable insulation shield materialwhich includes a chemical foaming agent over the insulation layer; andafter the extruding, performing the following steps: heating theconductor shield, the insulation layer and the insulation shield to atemperature equal to or greater than the decomposition temperature ofthe chemical foaming agent to decompose the chemical foaming agent;crosslinking the insulation shield material; crosslinking the insulationlayer; crosslinking the conductor shield; and foaming the insulationshield.
 17. The method of claim 16, wherein the chemical foaming agenthas a decomposition temperature and a processing temperature.
 18. Themethod of claim 17, wherein said extruding step is done at a temperatureless than the decomposition temperature of the chemical foaming agent.19. The method of claim 17, wherein the chemical foaming agent is anexothermic foaming agent and wherein the insulation shield materialfurther comprises a catalyst which lowers the decomposition temperatureof the chemical foaming agent.
 20. The method of claim 17, wherein theinsulation shield is maintained within the processing temperature rangeof the foaming agent in said heating step for at least about 1 minute.21. The method of claim 16, wherein said heating step is done at greaterthan atmospheric pressure.
 22. The method of claim 21, wherein saidheating step is done at a pressure greater than about 135 psi.
 23. Themethod of claim 16, further comprising: cooling the electrical powercable after said foaming step.
 24. The method of claim 23, furthercomprising applying a metallic shield over the foamed insulation shieldafter said cooling step.
 25. The method of claim 16, wherein saidheating step is done at about 600° F. to about 750° F.
 26. The method ofclaim 16, wherein said heating step is done at about greater than 370°F.
 27. The method of claim 16, wherein said three extruding steps aredone simultaneously.
 28. The method of claim 16, wherein said threecrosslinking steps and said foaming step are done substantiallyconcurrently.
 29. A method of producing an electrical power cable,comprising: advancing an electrical conductor through an extrusioncrosshead; extruding a semiconductive conductor shield over theelectrical conductor; extruding a cross-linkable electrical insulationlayer over the conductor shield; extruding a semiconductive,crosslinkable insulation shield material which includes a chemicalfoaming agent having a decomposition temperature and a processingtemperature range, over the insulation layer, said extruding being doneat a temperature less than the decomposition temperature of the chemicalfoaming agent; and after the extruding, performing the following steps:heating the electrical power cable with the conductor shield, theinsulation layer and the insulation shield to a temperature equal to orgreater than the decomposition temperature of the chemical foaming agentto decompose the chemical foaming agent, said heating being done atgreater than atmospheric pressure; crosslinking the insulation shieldmaterial; crosslinking the insulation layer; crosslinking the conductorshield; and foaming the insulation shield.
 30. The method of claim 29,further comprising: cooling the electrical power cable after saidfoaming step; and applying a metallic shield over the foamed insulationshield after said cooling step.
 31. The method of claim 29, wherein saidheating step is done at a pressure greater than about 135 psi.
 32. Themethod of claim 29, wherein the chemical foaming agent is an exothermicfoaming agent and wherein the insulation shield material furthercomprises a catalyst which lowers the decomposition temperature of thechemical foaming agent.
 33. The method of claim 29, wherein said heatingstep is done at about 600° F. to about 750° F.
 34. The method of claim29, wherein said heating step is done at about greater than 370° F. 35.The method of claim 29, wherein said three extruding steps are donesimultaneously.
 36. The method of claim 29, wherein said threecrosslinking steps and said foaming step are done substantiallyconcurrently.
 37. The method of claim 29, wherein the insulation shieldis maintained within the processing temperature range of the foamingagent in said heating step for at least about 1 minute.